For a conventional surface emitting laser, a structure shown in FIG. 6 is well known. More specifically, a lower multilayer reflection film 42 which comprises laminated semiconductor layers having different refractive indexes, called DBR (distributed Brag reflector) and reflects light with a specific frequency band is formed on a semiconductor substrate 41 made of GaAs, for example. Then, a light emitting layer forming portion 46 comprising a lower spacer layer, an active layer and an upper spacer layer (not shown) and an upper multilayer reflection film 48 comprising DBR are formed thereon and then an insulating region 47 is formed by implanting ion such as proton to a part other than a current injection region serving as the light emitting region.
Upper and lower electrodes (not shown) are provided on the upper surface of the upper DBR and lower surface of the semiconductor substrate and laser beam is output from a small outlet provided in the upper electrode (not shown) through the upper multilayer reflection film 48 whose reflectance is made a little smaller.
In addition, recently, in stead of insulation by ion implantation of proton or the like, there is proposed a method of forming the current limit layer comprising a step of intervening a semiconductor layer for selective oxidization made of AlAs, for example which is likely to be oxidized by vapor or the like between the light emitting layer forming portion 46 and the upper multilayer reflection film 48, a step of etching its peripheral part such that the current injection region and a certain region of its outer periphery may be left in mesa-shape, a step of selectively oxidizing it from its outer exposed part under a vapor atmosphere and a step of stopping the oxidation treatment so as not to oxidize the current injection region.
According to the conventional method in which a predetermined range of current injection region is formed in the center by stopping the oxidation treatment, a test sample is previously oxidized on a certain condition, that sample is taken out of the oxidation furnace, an oxidized distance is measured and then, the oxidation treatment is performed for a certain period based on that data to form the predetermined current injection region.
As described above, if the semiconductor layer for selective oxidation which is likely to be oxidized is provided to form the semiconductor laminated part and then the current limit layer is formed such that the desired size of current injection region may be left by oxidizing the semiconductor layer for selective oxidation from its periphery, a current can be injected into a narrow region without a different kind of step in which the semiconductor substrate is taken out of an equipment to provide a mask on the semiconductor layer in the course of lamination of the semiconductor layers and etched, or a complicated step in which the semiconductor layer around the current injection region is insulated by ion implantation using a mask after the semiconductor laminated part is formed.
As described above, the method of forming the current limit layer by performing selective oxidation treatment after the semiconductor laminated part is formed using the semiconductor layer for selective oxidation is simple and preferable for the manufacturing steps. However, even if the oxidation condition is set using the test sample, oxidation speed is likely to be changed by variation in temperature of the substrate put in the oxidation furnace, change in partial pressure or temperature of vapor, or variation in thickness of the current limit layer. Thus, since it is necessary to perform the condition setting using the sample several times, it takes time, and yet large variation still exists in practice.
Especially, in a case of the surface emitting laser, a width of the current injection region is about 8 μm square in a multimode oscillation as is used conventionally and its variation is about ±1 μm to 2 μm, which does not matter. However, in a single-mode oscillation to be required from here on, the current injection region needs to be 3.5 μm square or less depending on its wavelength but if it becomes too small, the output is extremely decreased and therefore its variation must be under 0.5 μm. Thus, in the conventional method of performing oxidation treatment by setting a certain condition, the yield ratio is decreased, which is not practical.
Furthermore, as a result of the earnest investigation by the inventor of the present invention, it was found that oxidation time is in proportion to oxidation distance at the beginning of the oxidation but the proportionality relation is destroyed when the width of the region not to be oxidized and to be left as the current injection region is 5 μm or less, that is, the oxidation is more progressed for the same period, so that in a case where the small current injection region for the single-mode oscillation is to be formed, the oxidation treatment performed by previously setting the condition cannot be precisely controlled.